Biogeochemical Fe-Redox Cycling in Oligotrophic Deep-Sea Sediment

Biogeochemical redox cycling of iron (Fe) essentially governs various geochemical processes in nature. However, the mechanistic underpinnings of Fe-redox cycling in deep-sea sediments remain poorly understood, due to the limited access to the deep-sea environment. Here, abyssal sediment collected from a depth of 5800 m in the Pacific Ocean was characterized for its elemental, mineralogical, and biological properties. The sedimentary environment was determined to be oligotrophic with limited nutrition, yet contained a considerable amount of trace elements. Fe-redox reactions in sediment progressed through an initial lag phase, followed by a fast Fe(II) reduction and an extended period of Fe(III) oxidation before achieving equilibrium after 58 days. The presence of an external H2 electron donor significantly increased the extent of Fe(III) bio-reduction by 7.73% relative to an amendment-free control under high pressure of 58 MPa. A similar enhancement of 11.20% was observed following lactate amendment under atmospheric pressure. Fe(II) bio-oxidation occurred after 16 days’ anaerobic culturing, coupled with nitrate reduction. During Fe bio-redox reactions, microbial community composition was significantly shaped by the presence/absence of an electron donor, while the hydrostatic pressure levels were the controlling factor. Shewanella spp. emerged as the primary Fe(III)-reducing microorganisms, and were stimulated by supplemented lactate. Marinobacter hydrocarbonoclasticus was the predominant Fe(II)-oxidizing microorganism across all conditions. Our findings illustrate continuous Fe-redox reactions occurring in the deep-sea environment, with coexisting Fe-redox microorganisms determining the oscillation of Fe valence states within the abyssal sediment.